The present invention relates generally to a method for purifying carbon dioxide gas. In particular, the present invention relates to a method for removing sulfur dioxide (SO2) from carbon dioxide gas comprising SO2 as a contaminant. The method also removes NOx, if present as a further contaminant, from the carbon dioxide gas. The invention has particular application in the purification of crude carbon dioxide, e.g. flue gas from an oxyfuel combustion process in a pulverized coal fired power station in which sulfur containing carbonaceous or hydrocarbon fuel is combusted in a boiler to produce steam for electric power generation.
The term “SOx” means oxides of sulfur and includes SO2 and sulfur trioxide (SO3). The term “NOx,” means oxides of nitrogen and includes primarily nitric oxide (NO) and nitrogen dioxide (NO2). NOx may comprise one or more other oxides of nitrogen including N2O, N2O4 and N2O3.
It has been asserted that one of the main causes of global warming is the rise in greenhouse gas contamination in the atmosphere due to anthropological effects. The main greenhouse gas which is being emitted, carbon dioxide (CO2), has risen in concentration in the atmosphere from 270 ppm before the industrial revolution to the current figure of about 378 ppm. Further rises in CO2 concentration are inevitable until CO2 emissions are curbed. The main sources of CO2 emission are fossil fuel fired electric power stations and from petroleum fuelled vehicles.
The use of fossil fuels is necessary in order to continue to produce the quantities of electric power that nations require to sustain their economies and lifestyles. There is, therefore, a need to devise efficient means by which CO2 may be captured from power stations burning fossil fuel so that it can be stored rather than being vented into the atmosphere. Storage may be deep undersea; in a geological formation such as a saline aquifer; or a depleted oil or natural gas formation. Alternatively, the CO2 could be used for enhanced oil recovery (EOR).
The oxyfuel combustion process seeks to mitigate the harmful effects of CO2 emissions by producing a net combustion product gas consisting of CO2 and water vapour by combusting a carbonaceous or hydrocarbon fuel in pure oxygen. This process would result in an absence of nitrogen (N2) in the flue gas, together with a very high combustion temperature which would not be practical in a furnace or boiler. In order to moderate the combustion temperature, part of the total flue gas stream is typically recycled, usually after cooling, back to the burner.
An oxyfuel process for CO2 capture from a pulverised coal-fired power boiler is described in a paper entitled “Oxy-combustion processes for CO2 capture from advanced supercritical PF and NGCC power plants” (Dillon et al; presented at GHGT-7, Vancouver, September 2004), the disclosure of which is incorporated herein by reference.
Oxyfuel combustion produces raw flue gas containing primarily CO2, together with contaminants such as water vapour; “non-condensable” gases, i.e. gases from chemical processes which are not easily condensed by cooling, such as excess combustion oxygen (O2), and/or O2, N2 and argon (Ar) derived from any air leakage into the system; and acid gases such as SO3, SO2, hydrogen chloride (HCl), NOx and NO2 produced as oxidation products from components in the fuel or by combination of N2 and O2 at high temperature. The precise concentrations of the gaseous impurities present in the flue gas depend on factors such as on the fuel composition; the level of N2 in the combustor; the combustion temperature; and the design of the burner and furnace.
In general, the final, purified, CO2 product should ideally be produced as a high pressure fluid stream for delivery into a pipeline for transportation to storage or to site of use, e.g. in EOR. The CO2 must be dry to avoid corrosion of, for example, a carbon steel pipeline. The CO2 impurity levels must not jeopardise the integrity of the geological storage site, particularly if the CO2 is to be used for EOR, and the transportation and storage must not infringe international and national treaties and regulations governing the transport and disposal of gas streams.
It is, therefore, necessary to purify the raw flue gas from the boiler or furnace to remove water vapour; SOx; NOx; soluble gaseous impurities such as HCl; and “non-condensable” gases such as O2, N2 and Ar, in order to produce a final CO2 product which will be suitable for storage or use.
In general, the prior art in the area of CO2 capture using the oxyfuel process has up to now concentrated on removal of SOx and NOx upstream of the CO2 compression train in a CO2 recovery and purification system, using current state of the art technology. SOx and NOx removal is based on flue gas desulfurization (FGD) schemes such as scrubbing with limestone slurry followed by air oxidation producing gypsum, and NOx reduction using a variety of techniques such as low NOx burners, over firing or using reducing agents such as ammonia or urea at elevated temperature with or without catalysts. Conventional SOx/NOx removal using desulfurization and NOx reduction technologies is disclosed in “Oxyfuel Combustion For Coal-Fired Power Generation With CO2 Capture—Opportunities And Challenges” (Jordal et al; GHGT-7, Vancouver, 2004). Such process could be applied to conventional coal boilers.
U.S. Pat. No. 4,781,902 discloses a process in which SO2 and NOx may be removed from flue gas from combustion processes by a Selective Catalytic Reduction (“SCR”) of NOx to N2 (“deNOx”) using ammonia, followed by an oxidation of SO2 to SO3 using vanadium pentoxide-based catalysts. Water vapour reacts with SO3 and is then condensed within a Wet gas Sulfuric Acid (WSA) condenser to produce an aqueous solution of sulphuric acid. The deNOx and SO2-oxidation reactions take place at elevated temperature, typically about 400° C., and at about atmospheric pressure. 90-95% SO2 conversion using this process has been calculated to require a volumetric hourly space velocity in the SO2 oxidation reactor of about 2500 Nm3feed/h/m3catalyst bed. The process has been developed by Haldor Topsøe A/S of Lyngby, Denmark and is known as the SNOX™ process.
US 2004/0071621 A1 discloses a process for the removal of SO2 from flue gas generated in a combustion process. The flue gas is cooled and soot is removed from the cooled flue gas. The cooled soot-free flue gas is further cooled and SO2 in the further cooled, soot-free, flue gas is oxidized to SO3 in the presence of O2 using a catalyst unit containing structured arrangements of at least one activated carbon fiber board, and is washed with water to produce a dilute (2˜5%) aqueous solution of sulfuric acid. The washed gas is then discharged directly to the atmosphere via a mist eliminator (which is optional) and a smoke stack. It is disclosed that the pressure of the flue gas is elevated to cause the gas to pass through the catalyst unit.
US 2007/0122328 A1 (granted as U.S. Pat. No. 7,416,716 B1) discloses the first known method of removing SO2 and NOx from crude carbon dioxide gas produced by oxyfuel combustion of a hydrocarbon or carbonaceous fuel, in which the removal steps take place in the CO2 compression train of a CO2 recovery and purification system. This process is known as a “sour compression” process since acid gases are compressed with carbon dioxide flue gas. The method comprises maintaining the crude carbon dioxide gas at elevated pressure(s) in the presence of O2 and water and, when SO2 is to be removed, NOx, for a sufficient time to convert SO2 to sulfuric acid and/or NOx to nitric acid; and separating said sulfuric acid and/or nitric acid from the crude carbon dioxide gas.
There is a continuing need to develop new methods for removing SO2 and, where present, NOx from carbon dioxide gas, and particularly from crude carbon dioxide gas such as flue gas produced in an oxyfuel combustion process such as that involved in a pulverized coal-fired power boiler.